This application relates to three-roll presses, and more particularly to a pressure compensated single nip three-roll press for calendering or pressing web material and the like, with a uniform force across the nip.
A particular use for such presses is the fixing of a toner image on a toner image support member as shown in Tsunoi et al, U.S. Pat. No. 4,192,229 issued Mar. 11, 1980; Sasaki, U.S. Pat. No. 4,343,234 issued Aug. 10, 1982; and Sasaki, Re. No. 32,126 reissued Apr. 29, 1986. A further particular application of a roll-type press, in which a uniformly high pressure is required along a nip between a pair of pressing rolls, is the rupturing of photosensitive microcapsules on a surface coating carried on a web or sheet of material.
U.S. Pat. Nos. 4,440,846 and 4,339,209, assigned to the same assignee as this invention and incorporated herein by reference, describe an imaging system wherein a photosensitive layer, comprising microcapsules containing a photosensitive composition in the internal phase, is image-wise exposed to actinic radiation and subjected to a uniform rupturing force whereupon the microcapsules rupture and image-wise release the internal phase. The imaging system is particularly advantageous because it is a totally dry system and does not rely upon the application of wet developing processing solutions to produce the image. An imaging-forming chromogenic material, such as a substantially colorless former, is typically associated with the microcapsules.
When the microcapsules rupture, the color former image-wise reacts with the developer material and produces a color image. In the embodiments described in the referenced patents, the microcapsules are typically ruptured by passing image-wise exposed imaging sheets through the nip between a pair of parallel calender rolls.
The media may exist in either single sheet or two-sheet versions. In the former, the microcapsules and developer composition are both coated onto a single substrate layer. In the latter, the microcapsules are carried on a first substrate layer, referred to as a donor sheet. The developer composition is coated onto a second, separate substrate layer, referred to as a receiver sheet. The donor sheet is subjected to the actinic radiation, and the exposed microcapsule layer is then brought into contact with the developer layer of the receiver sheet. The two sheets are then developed by pressure, with the finished image being formed in the receiver sheet.
Skewed rolls have been suggested for use in such apparatus. However, unsatisfactory results can be obtained where the skewed roll comes into contact with the image bearing material, since the skew in such a roll, satisfactory to achieve a sufficient force and a force distribution, can create an excessive shear in the media which can degrade the developed image. In order to avoid unwanted shear, four-roll developers have been suggested, employing two uniform diameter nip-forming pressing rolls, and a pair of crowned compensating rolls. The rolls of uniform diameter provide a constant velocity for the media bearing material at the nip, while the crowned rolls provide a uniform force when properly deflected, against the uniform rolls. While this concept is capable of producing superior results, the use of four rolls increases complexity, weight and cost.
Three-roll presses are disclosed in the above-identified U.S. Pat. Nos. 4,192,229; 4,343,234 and Re. No. 32,126. In each case a pair of cylindrical, parallel, nip-defining pressing rolls are employed together with a third compensating roll, the latter being either crowned or mounted on an axis skewed to the parallel rolls. The intermediate roll, that is the roll between the compensating roll and a first roll, is allowed to float, although it may be journalled or restrained at its ends so that it remains in parallelism with its nip-defining first roll, when loaded by the compensating pressure roll. Such arrangements, as defined, are deficient in that they fail to take into account the fact that the deflection curves between the parallel pressing rolls cannot be accurately matched at regions near the edges of the web, regardless of loading, as long as the intermediate roll is free-floating at its ends.